1. Field of the Invention
The present invention relates in general to an air-fuel ratio control system for an internal combustion engine, and more particularly to an air-fuel ratio control system which is applied to internal combustion engines of a type which has an evaporative emission control system.
2. Description of the Prior Art
In order to prevent the escape of fuel vapors from the fuel tank and the intake system of an internal combustion engine, evaporative emission control systems (EECS) have been widely employed in modern motor vehicles. In the systems, an activated charcoal canister is used to trap the vapors when the engine is shut off. Upon restarting, a flow of filtered air through the canister purges the vapors from the canister. The vapors go through one or more tubes (purge line) feeding into an induction passage downstream of a throttle valve of the intake system, and they are burnt in the engine.
In an engine controlled by a so-called "air-fuel ratio feedback control system", the vapors introduced into the induction passage tend to disturb the air-fuel ratio of the mixture has been previously adjusted by the control system. In order to deal with this undesired disturbance, various measures have been hitherto proposed and put into practical use. Some of them are disclosed in Japanese Patent First Provisional Publication Nos. 57-86555 and 57-129247.
In these measures, an electromagnetic valve is connected to the purge line to control the amount of the vapors supplied to the induction passage from the charcoal canister in accordance with an information signal issued from an air-fuel ratio sensor disposed in the exhaust system of the engine. That is, the valve is of a type in which the valve open degree increases in proportion to a duty value (viz., the rate of the time for which the valve opens to the entire time for which the same effects the open and close cycles). In the measures, the duty value based on an air induction rate is corrected in accordance with the information signal from the air-fuel ratio sensor thereby to put the influence of the disturbance in a controlled range.
When, for example, the engine is restarted after long standstill, the initially purged vapors from the charcoal canister contain a larger amount of fuel, so that the air-fuel ratio previously set by the air-fuel ratio control system is forced to deviate from a desired value (viz., stoichiometric value) causing the air-fuel mixture actually burned in the engine to become rich. Upon this, the air-fuel ratio sensor in the exhaust system issues a signal representing that the mixture has become richer than stoichiometric by a degree corresponding to the amount of the rich vapors, and the duty value is decreased for reducing the amount of the vapors fed to the induction passage. With this, the air-fuel ratio of the mixture is returned to the desired value.
In general, the air-fuel ratio feedback control is used only in a feedback control zone wherein the substantive air-fuel ratio of the mixture actually supplied to the engine can be controlled within a predetermined range which includes a desired air-fuel ratio as a base value. This is because using the feedback control zone can deal with not only a relatively large change in the air-fuel ratio but also a requirement for achieving a relatively stable control of the air-fuel ratio. That is, the feedback control zone is a balanced zone in which the above-mentioned two matters are achieved at the same time.
Apart from the above, the amount of fuel contained in the purged vapors changes largely in accordance with the time for which the engine has been at standstill and the temperature at which the engine (namely, associated vehicle) has been kept. Thus, it has been inevitably necessary to match the air-fuel ratio control range with the air-fuel ratio of the mixture which is prepared based on the engine standstill time and the engine temperature. Thus, when, after a long standstill, the engine is restarted and the operation of the engine is brought into the feedback control zone, the larger amount of fuel inevitably contained in the initially purged vapors causes the air-fuel mixture in the induction system to become rich instantly, so that the correction value to the feedback control exceeds the limit of the control range.
This will be understood from FIGS. 8, 9 and 10 of the attached drawings, in which the correction value to the feedback control is denoted by ".alpha.". Hereinafter, the correction value to the feedback control will be referred to as "feedback correction value".
FIG. 8 shows the waveform of the feedback correction value ".alpha." which is used for a proportional-plus-integral control. As is seen from this waveform, usually, the correction value ".alpha." varies between the maximum value ".alpha.MAX" and the minimum value ".alpha.MIN" of the control range. However, when the engine is under the above-mentioned air-fuel ratio feedback control carried out just after a long standstill thereof, the air-fuel ratio is forced to greatly change and thus the correction value ".alpha." exceeds the control range. In this condition, the value ".alpha." adopts the upper or lower limit value of the range in place of a calculated value, so that as is seen from FIGS. 9 and 10, the feedback correction value ".alpha." takes the maximum or minimum value ".alpha.MAX" or ".alpha.MIN" thereafter. This means that a substantial feedback control is impossible any longer thereby bringing about deterioration of composition of the exhaust gases from the engine.